In recent years, the increasing demand for higher density recording has led to wide use of a ferromagnetic powder suitable for high density recording in the magnetic recording medium.
However, if a magnetic recording medium having a magnetic layer with a smooth surface is employed, an increase in the coefficient of friction thereof with the recordin apparatus results, to thereby tend to damage the magnetic layer, or the magnetic layer is peeled off the recording medium after a short use.
In particular, the magnetic layer of a video tape is subjected to severe conditions such as still mode operation in a video tape player. If a magnetic recording medium is repeatedly used under such conditions, the ferromagnetic powder tends to separate from the magnetic layer, to thereby clog the magnetic head.
In order to improve the running durability of a magnetic recording medium, it has been proposed to incorporate into the magnetic layer of the recording medium an abrasive (hard particles) such as corundum, silicon carbide and chromium oxide. However, if the abrasive is incorporated into the magnetic layer, the abrasive must be used in a relatively large amount to obtain the desired effect.
However, a magnetic layer comprising a large amount of an abrasive causes a remarkable abrasion of the magnetic head or the like. Furthermore, this approach runs counter to the teaching of the art that the magnetic layer is to be made as smooth as possible in order to improve the electromagnetic properties. Thus, the use of an abrasive is not a desirable approach to improve the running durability.
In another approach, it has been proposed to incorporate a fatty acid or an ester of a fatty acid with a fatty alcohol into the magnetic layer as a lubricant to reduce the coefficient of friction. In particular, a higher saturated fatty acid which greatly reduces friction has been widely used in video tape systems.
Various combinations of lubricants have heretofore been studied, including, for example, the fatty acid esters-fatty acid amide mixture systems disclosed in JP-B-47-15624 (the term "JP-B" as used herein means an "examined Japanese patent publication") and fatty acid ester mixture systems as disclosed in JP-B-51-39081. However, these lubricants stain the running path due to separation of the power from the magnetic layer, increase the running tension under conditions of high humidity, result in a deterioration in repetitive running performance and an increase in the drop out. In order to eliminate these disadvantages, it has been proposed to provide on the surface of the magnetic layer at least one C.sub.12-22 saturated or unsaturated fatty acid, at least one saturated or unsaturated fatty acid ester containing a C.sub.14-22 fatty acid residue, and at least one C.sub.12-26 saturated or unsaturated fatty acid amides as disclosed in JP-B-59-3981. It was confirmed that this approach can remarkably eliminate the above described disadvantages.
However, particularly in more recent systems wherein a ferromagnetic powder is desired to be highly dispersed to obtain excellent electromagnetic properties, the use of saturated fatty acids reduces the dispersibility of the ferromagnetic powder or causes the agglomeration thereof, and thus cannot be incorporated into the magnetic layer in as large an amount as in the above-described techniques. Furthermore, the use of saturated fatty acids causes an increase in the coefficient of friction under conditions of high temperature and humidity. Thus, the above-described approach is also unsatisfatory.
Furthermore, it is known that in a system comprising a ferromagnetic metal powder, the addition of saturated fatty acids as a lubricant or as a magnetic layer component causes staining of the calendering roll used at the step of smoothening the magnetic recording medium, thereby resulting in poor productivity.
Moreover, JP-A-61-230624 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposed a disc magnetic recording medium comprising a magnetic layer containing therein a fatty alkyl ester and a fatty alkoxy ester. It was found that this approach provides a remarkable improvement in the durability. However, this approach is disadvantageous with respect to an increased coefficient of friction and staining of the calender rolls.
The present inventors have conducted extensive studies to provide lubricants which eliminate the above described disadvantages. As a result, it has been discovered that a magnetic recording medium excellent in electromagnetic properties and running performance is achieved, and which is efficiently manufactured without staining the calender rolls by incorporating in the magnetic layer containing a ferromagnetic powder at least one fatty acid amide compound and two specific fatty acid esters compounds having differen structural formulae in combination, to thereby achieve the present invention.